The present invention relates to processes in multi-stage gas phase oxidation reactors for controlling particle sizes and particle size distribution of titanium dioxide.
Titanium dioxide, TiO2, is well known as the premier pigment for a variety of applications, including paints, coatings, plastics and other such products. The optical properties of TiO2 pigments, in particular light scattering characteristics, are strongly dependent on pigment particle size and particle size distribution. Optimum pigmentary performance (opacifying and thus aesthetic properties of the pigment) is typically achieved when the mean TiO2 particle size is between about 0.2 to about 0.3 xcexcm and the particle size distribution is typically a log normal or geometric distribution having a standard deviation of less than about 1.50.
Two additional pigment-related optical properties are undertone and gloss. These properties are also functions of particle size and distribution. For example, gloss, a particularly important property of certain paint systems using TiO2 pigments, is adversely affected by particulates larger than about 0.5 xcexcm. Additionally, the reflective, or undertone values, of small particles impart a desirable bluish tone in paint systems.
Significant effort has gone into learning how pigments are formed and how to control their manufacture. The prior art is replete with methods purported to solve problems associated with particle size control while maintaining acceptable production rates.
A common process for manufacturing useful rutile titanium dioxide is by the so-called xe2x80x9cChloride Processxe2x80x9d which involves a reaction between titanium tetrachloride (TiCl4) vapor and an oxidizing gas, such as air, oxygen or mixtures of such oxidants, to produce particles of TiO2. For example, Equation (I) describes the overall reaction scheme:
TiCl4+O2xe2x86x92TiO2+2Cl2xe2x80x83xe2x80x83(I)
While not being bound to any one particular theory, in the Chloride Process, TiO2 particles are believed to form by a sequence of three basic steps:
(1) formation of TiO2 nuclei;
(2) nuclei growth to form crystals or single, primary particles; and
(3) Single or primary particle agglomeration to form larger clusters of particles wherein the primary particles are held together through various degrees of sintering.
It is well known that the oxidation reaction (I) has a high activation energy. Thus, a significant amount of energy is required to preheat the reactants to initiate the reaction converting TiCl4 and an oxidant into pigmentary TiO2 and to obtain stable reactant conversions. Further, since reaction (I) is also highly exothermic, there is a large increase in the temperature of the efflux stream containing TiO2 from the reactor. Therefore, when substantially all of an initial fraction of a TiCl4/oxidant charge reacts, there is a large heat release causing a significant rise in reactor/reaction mass temperature which increases the rate of the reaction and causes the TiO2 particle growth rate in the reaction mixture to increase. Further, in processes such as the Chloride Process where high throughputs of reactants at high pressures are used, the ability to satisfactorily cool reaction products is reduced or minimized by the temperature increase. Thus, the ability to limit and control the growth of the TiO2 particles to the optimum size range of from about 0.2 to 0.3 xcexcm, while maintaining an acceptable particle size distribution, is lost as higher temperatures result from higher throughputs.
The high temperature conversion of reactants into products tends to lead to large agglomerates of sintered primary TiO2 particles, which are generally detrimental in pigmentary applications. Strongly agglomerated particles, where the agglomerate sizes are greater than about 0.5 xcexcm, typically result when high temperatures and high throughputs are encountered. In order to obtain particle sizes and particle size distributions useful as commercial pigment, energy intensive milling is required.
One variation of producing titanium dioxide via the Chloride Process relies on introducing reactants, titanium tetrachloride and oxidants, through multiple inlets into a reactor. These types of reactors are generally referred to as xe2x80x9cmulti-stage oxidation reactorsxe2x80x9d and they are typically designed to allow the heat of reaction generated in one reactor stage to be used to heat the reactants in a subsequent stage thereby improving the economy of operation. Patents describing variations on the multi-stage process of producing pigmentary rutile TiO2 include: U.S. Pat. No. 3,463,610; U.S. Pat. Nos. 4,803,056; 5,599,519; U.S. Pat. No. 5,840,112; and GB Patent 2,037,266B.
U.S. Pat. No. 3,463,610 discloses a process for the production of particulate TiO2 facilitated by the introduction of particulate matter to seed the formation of pigments. The process discloses introducing reactants into the reaction zone through at least two inlets that are spaced long a reaction zone. It is specifically desirable that substantially all of he reactants (or reactant) introduced into the gas stream in the reaction one react completely before that portion of the gas stream reaches the next inlet into the reaction zone where another introduction of reactant(s) is made. Means or steps for ensuring substantially complete reaction of the reactants in each reactor stage are disclosed.
U.S. Pat. No. 4,803,056 describes a multi-stage reactor for increasing production capacity and utilizing the heat of reaction in a multi-stage process for producing TiO2. The patent compares the output of a single stage reactor to that of a similar sized two-stage reactor. The multi-stage reactor relies on splitting the total flow of TiCl4 and subsequently completely reacting it with excess oxygen. The excess heat in the first reactor stage is then used to heat the remainder of the TiCl4 added in the final reactor stage.
U.S. Pat. No. 5,599,519 describes a multi-stage process for producing titanium dioxide which avoids the need for auxiliary heating of the reactants using known means such as hydrocarbon combustion or electric discharge. The patent discloses that complete reaction of titanium tetrachloride in each stage should be achieved prior to adding any additional TiCl4 in a subsequent reactor stage.
An objective of each of these patents is to react substantially all of the TiCl4 added in each stage of the multi-stage reactor prior to passing the reacted mass into a subsequent reactor stage where any additional reactants may be added.
U.S. Pat. No. 5,840,112 additionally discloses a multi-stage process for producing TiO2 where oxygen is separately introduced in significantly less than stoichiometric quantities into each stage of the reactor and at differing temperatures to control the reaction. This patent also describes secondary additions of TiCl4, but does not disclose limited lo conversion of TiCl4 in the presence of excess oxygen, nor the control of TiCl4 conversion within the reactor in order to affect the particle size distribution of titania produced.
Thus, based on the prior art, there still exists a need for improved methods of producing titania pigments which do not yield excessive quantities of undesirably sintered primary particles which typically result from a substantially complete conversion of reactants in a particular stage of a multi-stage reactor.
An objective of the present invention is to provide a process for producing TiO2 having fewer, smaller, or more easily milled titania agglomerates, while potentially increasing the production rate of pigment by controlling the extent or degree of conversion of titanium tetrachloride into titanium dioxide within a first or intermediate reactor stage.
A second objective is to provide a stable yet flexible process for producing TiO2 in a multi-stage gas phase reactor wherein there are multivariate controls over the reaction profile within the reactor. A third objective of the invention is to provide an independent means for flexible reaction temperature profile control in a second or subsequent stage of a reactor when practicing the methods of the present invention.
With a view to these objectives, it has been discovered surprisingly that by actively controlling the percentage of TiCl4 conversion in a first or intermediate reaction zone, prior to a subsequent reaction zone, the production of optimum particle sized TiO2 may be achieved, thereby reducing the need for expensive, time consuming, energy intensive milling steps.
In other embodiments, the present invention provides processes for producing titanium dioxide in multi-stage gas phase reactors wherein the growth of the titanium dioxide particles is limited. The processes comprising reacting in a first stage of the reactor only a fraction of the titanium tetrachloride of an initial gaseous mixture comprising titanium tetrachloride with an oxygen-containing gas. This reaction forms a reaction mixture comprising titanium tetrachloride, titanium dioxide, chlorine and oxygen. Further, this reaction is effected under conditions such that the reaction mixture is resident in a first, or intermediate reactor stage for a mean residence time that limits the growth of the titanium dioxide particles produced. Thereafter, in subsequent stages of the reactor, additional gaseous mixtures comprising titanium tetrachloride may be reacted with the reaction mixture from a preceding reactor stage.
In another embodiment the present invention comprises reacting in a first stage of a multi-stage reactor a fraction of an initial charge comprising titanium tetrachloride with an excess of an oxygen-containing gas under conditions that limit the first stage temperature to a maximum of not greater than about 1400xc2x0 C. Thereafter, in any subsequent stages of the reactor, titanium tetrachloride is reacted with an oxygen-containing gas under conditions that limit such subsequent stage temperatures to a maximum of not greater than about 1600xc2x0 C.
In yet another embodiment of the invention, liquid titanium tetrachloride is added to the initial gaseous mixture of titanium tetrachloride, added to a second or subsequent reactor stage, thereby forming an admixture having a lower temperature, prior to introducing the admixture into a second, or subsequent reactor stage. This process is termed xe2x80x9cdesuperheatingxe2x80x9d and may be accomplished using any of a variety of desuperheaters used in other applications assuming that the desuperheater is constructed so as to be adapted to a gas phase chloride reactor (e.g. materials of construction and the like). In a preferred embodiment, the process comprises combining the prior two embodiments.
Various alternatives may be used to facilitate the objectives of the invention. The following embodiments are presented to aid in an understanding of the present invention and are not intended to, and should not be construed to, limit the invention in any way. All alternatives, modifications and equivalents which may becomes obviousness to those of ordinary skill upon a reading of the present disclosure are included within the spirit and scope of the present invention.
As this disclosure is not a primer on TiO2 pigment production or on the design nor manufacture of oxidation reactors, basic concepts known to those skilled in the field of TiO2 production have not been set forth in detail. Concepts such as choosing appropriate reactor manufacturing materials, or appropriate additives for the oxidation reaction producing titania pigment, or suitable conditions for typical operation of oxidation-type reactors are readily determinable by those skilled in the industry and are generally described in the prior art. Attention is therefore directed to the appropriate texts and references known to those skilled in the art in regard to these matters.
Mean residence time (t) is basically a function of the volume of the reactor (V), and the volumetric flow rate of the reactants (Q), and may be simply stated as in Equation (II):
t=(Q/V)xe2x80x83xe2x80x83(II)
At a given temperature and pressure, the longer the mean residence time, the greater the conversion of TiCl4 to TiO2, until substantially 100% conversion is achieved. However, for a given mass of TiCl4 and oxygen, the amount of total heat liberated by the exothermic reaction between the two components is in fixed proportion to the amounts actually reacted.
In practice, the distribution of residence times within a reaction vessel is a complex function of mixing intensity, density of gases and temperature profiles. Further, since mixing is not instantaneous, there is a distribution of temperatures and reactant conversions across the reaction zone for a given mean residence time. These parameters may be calculated using equations well known in the art of fluid mechanics and kinetic theory.
To practice the present inventive process, the physical parameters of a first or intermediate reaction zone of a multiple stage reactor are adjusted for anticipated process conditions as described by Equation (II) to achieve the desired percent conversion of titanium tetrachloride at the end of that reaction zone. Generally, mean residence times of less than 30 milliseconds are preferred in the first or intermediate reaction zone, with times between 0.1 and 20 milliseconds being more preferred.
Once the mean residence time in a reacting stage has been shortened to less than that length of time required for substantially 100 percent conversion, the initial mixing rates between the reactants may be used to further adjust the extent of titanium tetrachloride conversion achieved by controlling the flow of TiCl4 into the reaction zone. The flow may be controlled by, for example, adjusting the width of the slots or orifices through which TiCl4 enters a reaction zone. As one of ordinary skill will understand, provided there is sufficient energy to drive the reaction rapidly, an increase in slot width will generally decrease the initial mixing rates of the reactants and broaden the distribution of conversion of the reactants across the reactor cross section. Decreased mixing will delay the reaction, which will decrease both the maximum temperature in the reactor and the time the newly formed titania is exposed to that temperature in that reaction stage.
It will be understood by those of skill in the art that a complete, quantitative conversion (100 percent) of reactants into products is difficult to achieve in practice. For the purposes of this disclosure, complete conversion of reactants is defined as reacting substantially all of a reactant added to a reactor or a reactor stage, e.g. approaching about 100 percent conversion, given the practical operating limitations of a particular reactor.
Incomplete conversion may be achieved by controlling the residence time distribution of the reactants in the reactor. For the purposes of this disclosure, the mean residence time is that time which would be required for the volume of the reactants to pass through a reactor stage if no reaction were to take place.
Partial conversion of titanium tetrachloride controls the liberation of reaction energy. It is believed that partial conversion tends to equilibrate the liberated reaction energy across the remaining greater mass of reacted and unreacted reagents and product within the reactor, the unreacted mass essentially acting as a heat sink, providing additional stabilizing control over the reaction. xe2x80x9cPartial conversionxe2x80x9d thus allows a substantial increase in the minimum reactor temperature (at which the reaction is run) over that minimum temperature which is required to minimally initiate the reaction converting TiCl4 into titanium dioxide. In contrast to the extreme temperatures and longer time periods involved in conventional processes, where complete conversion of the reactants occur, the xe2x80x9cpartial conversionxe2x80x9d process limits the time that newly formed titania particles see extremely high temperatures. This limits particle sintering that would otherwise be encountered with higher initiation temperatures.
Control of the mean residence time, and thus the extent of reactant conversion, may be easily achieved by manipulating combinations of variables well known to those of skill in the art. Those variables include, but are not limited to, the reactant concentrations, reactor pressures, flow rates of reactants, reaction zone dimensions, the turbulent mixing rates of incoming reactant streams, reactant temperatures and the like.
The mean temperature at the end of a first or intermediate reaction zone(s) should be at least about 900xc2x0 C., but less than about 1600xc2x0 C. Temperatures between about 1200xc2x0 C. to about 1600xc2x0 C. are more preferred. Specifically, incomplete conversion typically limits the maximum temperature to about 1200xc2x0 C. that is sufficient to ensure greater than 99 percent rutilization of the final product.
In practicing the present invention, partial conversions of between about 30% to about 70% of the TiCl4 added to each of a first or intermediate reaction zone(s) are preferred. It is more preferred that between about 40% to about 60% of the TiCl4 added to a first or intermediate reaction zone(s) be converted to TiO2 by the end of the reaction zone(s).
For a given set of conditions, higher temperatures and longer exposure times lead to numerous undesirably large particles having increased degrees of agglomeration. In particular, these agglomerates, with strong intra-agglomerate bonding, are a limiting factor in producing commercial quality pigment. Strongly bound agglomerates must be milled, and milling is expensive and limits overall process productivity. Thus, controlling the maximum reaction temperature and reducing the amount of time that newly formed TiO2 particles are subjected to high temperatures in the reactor are effective means of limiting particle and agglomerate growth.
With a view to practicing the present invention, any conventional type of corrosion resistant reaction vessel suitable for multi-stage operation may be employed. The vessel must be of such design, construction and dimension that a continuous flow of reactants and products within and through the oxidation chamber(s) will be afforded and such control over the velocities, mixing rates, temperatures, and thus residence times distributions, will be permitted. A typical reactor useful to practice the methods of the present invention may include a combustion chamber for preheating reactants and other such associated equipment as may be necessary for the safe operation of a multi-stage reactor to produce titania from TiCl4 and an oxidant according to the present invention.
The oxygen required for the reaction in the process of the present invention may be introduced into the first or subsequent zone of the multi-stage vapor-phase reactor by any suitable means, such as a stream of oxidizing gas from a combustion chamber. This stream may comprise air, oxygen-enriched air, or substantially pure oxygen. The total quantity of oxygen added must be sufficient to fully react with the total quantity of TiCl4 added to all of the zones of the reactor. The stream of oxidizing gas may be preheated by any variety of means well known in the art to a temperature between about 600xc2x0 C. to about 1000xc2x0 C. prior to being introduced into the combustion chamber. In a combustion chamber the oxidizing gas temperature may be further raised to between about 1300xc2x0 C. to about 1700xc2x0 C. by the combustion of a suitable fuel, for example a hydrocarbon fuel or other fuel known in the art. Other known means of raising the temperature also may be suitably employed. Depending on the preheating process employed, the stream of oxidizing gas feeding the reactor may be dry and relatively pure, but typically contains between about 50 PPM to about 200,000 PPM of water vapor based on the weight of TiO2 produced as would be known to the skilled artisan.
In processes of the present invention, TiCl4 is heated to typically, although not limited to, between about 250xc2x0 C. to about 450xc2x0 C., with temperatures between about 300xc2x0 C. to about 400xc2x0 C. being preferred.
A vaporizer or other such device well known in the art is useful for conditioning titanium tetrachloride. The vaporized TiCl4 is added to at least two or more inlet points in the reactor using means such as those disclosed by U.S. Pat. No. 3,512,219, the teachings of which are hereby incorporated by reference.
TiCl4 added to a first or subsequent reaction zone may be relatively pure, but is typically added as an admixture with aluminum chloride. The amount of aluminum chloride added to the reactor is such that the product comprises between about 0.1 to about 5 percent by weight of aluminum oxide. To achieve the desired admixture of TiCl4 and aluminum chloride for adding to the reactor, the preheated TiCl4 can be passed through any one of a number of types of aluminum chloride generators as described in the prior art. Passage through the aluminum chloride generator can be used also to raise the temperature of the TiCl4 to typically between about 375xc2x0 C. to about 660xc2x0 C.
It is advantageous to have the titanium tetrachloride admixture entering a second or subsequent stage be at a lower temperature, thereby having a beneficial effect on controlling particle growth and agglomeration. The temperature of the reaction mixture entering a second or subsequent stage will be reduced by the addition of cooler TiCl4 thereby controlling particle growth and agglomeration.
The temperature of the TiCl4 entering the second and subsequent stages can be generated at colder temperatures or from a single source reduced by any one of a number of heat exchange methods. However, in order to improve the process flexibility and impart operational stability to the process it has been suprisingly discovered that temperature reductions into a second or subsequent reactor stage may be beneficially achieved by passing the preheated TiCl4 gas feed through a xe2x80x9cdirect contact desuperheaterxe2x80x9d or xe2x80x9cattemperatorxe2x80x9d. Such devices are well known in the art of steam generation but have not been disclosed previously as means for controlling the temperature of a stream of TiCl4-containing gases being injected into a multi-stage chloride process reactor to produce titanium dioxide. A desuperheater useful in the present process may be as simple as a venturi-type device positioned inside a flow stream of titanium tetrachloride gas wherein the temperature of the hot titanium tetrachloride gases are moderated by evaporation of liquid TiCl4 which is supplied at a lower temperature through the desuperheater. The process of using a liquid to control the temperature of the vaporized mixture entering a second or subsequent stage of the reactor is termed xe2x80x9cdesuperheating.xe2x80x9d
Applying desuperheating technology to the Chloride Process confers numerous advantages, including low capital cost, simple and robust ease of process control, and the flexibility to control TiCl4 gas feed at any temperature between the normal process temperature and the saturation temperature of the gas, at the operating pressure selected. Additionally, because the desuperheater method takes maximum advantage of the latent heat of vaporization of the liquid titanium tetrachloride for cooling, a full range of temperatures may be achieved without unduly altering the concentration of any AlCl3 or other additive that might be added to each reactor stage feed. Thus, with this method of temperature control, potentially detrimental reductions of aluminum trichloride concentration in the titanium tetrachloride feed gas for second and subsequent stages can be minimized. If desired, a titanium tetrachloride solution containing dissolved aluminum trichloride may be supplied to the desuperheater providing additional process flexibility. Finally, by reducing the TiCl4 feed temperature from a higher level in this fashion, a robust process is obtained wherein an adequate range of temperatures are continuously available to a second or subsequent reactor stage to ensure the reaction stability and control to completion. A simple change of the liquid spray volume from the desuperheater is all that is required to restore or elevate reaction temperatures as necessary to control the correct percentage conversion of TiCl4. This feature improves the ability to operate the process close to the otherwise limiting lower process temperatures, hence allowing maximum benefit of the desuperheater technology to be realized.
Productivity is also enhanced by desuperheating the titanium tetrachloride gaseous feed into a second or subsequent stage of the reactor as illustrated by the examples. The extent of desuperheating useful in the present invention is between about 100xc2x0 C. to about 350xc2x0 C. and preferably between about 125xc2x0 C. to about 225xc2x0 C.
In the process of this invention, the multi-stage reactor is operated at a pressure above atmospheric pressure. In the preferred mode the reaction is carried out at a pressure of at least about 5 psig. In general the pressure within the reactor will be between about 5 psig to about 100 psig, although higher pressures are possible and become more attractive using the disclosed processes. In a preferred process, a pressure of about 60 psig is used.
The reaction may be carried out in the presence of useful alkali additives (for example, the halide salts of alkali metal) and water vapor as is described in U.S. Pat. No. 3,208,666, U.S. Pat. No. 5,201,949 or GB 2,037,266B. Growth retardants such as alkali metal halides, including lithium, sodium, potassium, rubidium and cesium halides, may be added to one of the reactants prior to contact in the first reaction zone or prior to introduction into subsequent stages. The amount of alkali metal halide added may be up to about 3000 ppm based on the weight of TiO2 produced. In a more preferred process, metal halide is added between a concentration of about 5 ppm to about 1000 ppm while the most preferred concentration range is between about 10 ppm to about 500 ppm.
The suspension of TiO2 in chlorine and other diluent gases formed in a reactor may be cooled in a flue device such as that disclosed in U.S. Pat. No. 2,657,979, the teachings of which are hereby incorporated by reference, or another equivalent device. Granular scouring particles, or xe2x80x9cscrubsxe2x80x9d (for instance, sodium chloride, sand or calcined TiO2) maybe added to the flue to scrape away deposits of TiO2 on the internal surface of the flue pipe as disclosed in U.S. Pat. Nos. 2,721,626 and 3,511,308, the teachings of which are incorporated by reference.
Cooled pigment is typically separated from chlorine and any other gases and collected for further processing by filtration or other gas-solid separation techniques as are known in the art.